The innovative path of tungsten extraction, from traditional precipitation to ion exchange

The chemical structure and classification of tungsten resin

Tungsten-containing resin belongs to ion exchange resin and its chemical structure consists of three parts: 
1. Framework (Matrix) 
The resin framework is a cross-linked copolymer of styrene and divinylbenzene (St-DVB). 
2. Physical form 
The tungsten-containing resin used in industry is usually in the form of spherical particles, with a particle size range of 0.315 to 1.25 mm. According to its pore structure, it can be classified into two types: gel type and macroporous type. 
3. Functional groups 
The functional groups are chemically bonded to the framework, and they are the core units that determine the selectivity of the resin. According to the chemical properties of the functional groups, tungsten resins are classified into the following two categories: 
Strongly basic anion exchange resin 
The functional group is a quaternary ammonium group, with the chemical structure being -N⁺(CH₃)₃Cl⁻ or -N⁺(CH₃)₃OH⁻. This group remains positively charged at any pH, thus the resin has exchangeability in both acidic and alkaline solutions; the exchange reaction can be represented by the following chemical equation:

(In the formula, R represents the resin framework) 
(2) Weakly basic anion exchange resin 
The functional group is a tertiary amine, with the chemical structure being -N(CH₃)₂. It does not ionize in water and requires protonation in an acidic medium to form -N⁺(CH₃)₂H in order to undergo anion exchange. The selectivity of weakly basic resins for tungsten is mainly reflected in their preferential adsorption of polymeric tungstate ions (such as HW₆O₂₁⁵⁻, H₂W₁₂O₄₀⁶⁻) under acidic conditions. 
Protonation reaction (activation in acidic medium) 
The resin does not dissociate in water. It needs to be protonated under acidic conditions to form exchange groups:

(If using the sulfuric acid system, it would correspond to HSO4- or SO42- type) 
② Adsorption-Exchange Reaction (Selective Adsorption of Polytungstate Anions) 
Under weakly acidic conditions (pH 2 - 4), tungstate ions polymerize into polytungstate anions, which then undergo exchange reactions with the anions on the resin. 
· Regarding HW6O215 - (adsorption chemical formula)

· Regarding H2W12O406 - (adsorption chemical formula)

Note: The actual form of tungstate ions in the solution is complex. The above is a simplified proportional exchange formula. Charge balance follows the principle of equivalent exchange (i.e., 1 mole of charge corresponds to the exchange of 1 mole of chloride ions). 
③ Regeneration reaction (alkali liquid desorption) 
Using a strong base (such as NaOH) to neutralize the protons on the resin, breaking the ionic bonds, causing the polytungstate anions to be eluted, while the resin returns to the free amine form:

Where An− represents the polymeric ion of polytungstate, with n equal to 5 or 6. 
After regeneration, the resin is in the form of free alkali (-N(CH₃)₂). When it is reused, the acidification and protonation process described in step 1 needs to be repeated. 
4. Physical and Chemical Foundation of Resin Adsorption Reaction 
The selectivity of resin for the adsorption of tungstate ions mainly depends on the following factors: 
(Ion charge and hydration radius) 
WO₄²⁻ is a divalent anion, and its electrostatic interaction with quaternary ammonium groups is stronger than that of monovalent anions (such as Cl⁻ and OH⁻). 
(2) Solution pH 
Under alkaline conditions (pH > 9), tungsten exists in the form of WO₄²⁻; in weakly acidic to neutral conditions (pH 5 - 8), tungsten undergoes polymerization to form polytungstate anions, which are more easily adsorbed by the resin due to their higher charge density. 
(3) Competitive effect of impurity ions 
The selectivity sequence of strong basic anion exchange resins for common ions is approximately as follows:

According to this sequence, impurities such as phosphorus, arsenic, and silicon can be partially separated during the adsorption stage. However, molybdenum and tungsten have similar chemical properties, making separation difficult. Usually, deep separation needs to be carried out during the desorption stage.